Final answer:
The first step in creating a Dihybrid Punnett square is to determine the allelic combinations for the gametes of each parent based on their genotypes for the two traits being analyzed.
Step-by-step explanation:
The first step to make a Dihybrid Punnett square is to determine the allelic combinations for the gametes of each parent. This involves identifying the genotypes of the parents for the two traits being studied. Once the genotypes are known, the different alleles for each gene are separated into all possible combinations as they would appear in the gametes following the principles of Mendelian inheritance.
For instance, if we have a pair of heterozygous parents for two traits (AaBb and AaBb), we list all possible allelic combinations for the gametes. These could be AB, Ab, aB, and ab for each parent. These combinations are then placed along the top and the side of the Punnett square to predict the genotypic and phenotypic ratios of the offspring.
Therefore, the initial step in constructing a Dihybrid Punnett square is identifying the parent's genotypes and determining all the possible gamete combinations resulting from their genotypes, which will form the basis for the layout of the Punnett square.